Macrolide antibiotics are characterized by the presence of a macrocyclic lactone ring, the aglycone (See generally Macrolide Antibiotics: Chemistry, Biology and Practice (S. Omura, ed., Academic Press, New York)). Attached to the aglycone are one or more deoxy sugars. The sugars may be acylated. The macrocyclic ring is commonly 12-, 14-, or 16-membered bum larger rings are also known. The mechanism of action of macrolide antibiotics involves the inhibition of protein synthesis.
The macrolide antibiotics are highly active against gram-positive organisms such as Staphylococcus, Streptococcus, and Diplococcus and also have activity against gram-negative organisms such as Neisseria Gonorrhea and meningitidis, Bordetella pertussis, and Haemophilus influenzae. Id. at p.26. All of the above strains are capable of causing significant illnesses. Macrolides, including spiramycin and tylosin, have been used clinically in the medical and veterinary fields due to their low toxicity. Id. at p.27.
Members of the macrolide family of compounds which are also referred to as macrocyclic lactones have utilities beyond antibiotic activity. For example FK506 has potent immunosuppressive activity and thus offers promise in therapeutic applications such as suppression of organ transplant rejection, rheumatoid arthritis, and various other autoimmune states. Other macrolides such as avermectin have activities including insecticidal and anti-helminthic activities.
Because the macrolides are so clinically useful, it is of the utmost importance to clone the genes responsible for producing the enzymes of the respective biosynthetic pathways. These genes can be used to increase the enzyme concentration in an organism, thereby increasing the efficiency of antibiotic production (Chater, 1990, Biotechnology 8: 115-121. The genes may be shuttled among various antibiotic producers to generate hybrid antibiotics, due to the "loose" substrate specificities of some of the biosynthetic enzymes (Sadakane et al., 1982, J. Anti-biotics 35:680-687; Hopwood 1989 Phil. Trans-R. Soc Lond. B 324: 549-562; Hutchison et al, 1989, Drug Discovery and Development Through The Energetic Engineering of Antibiotic--Producing Microorgansims, J. Med. Chem. 32: 929-937). In addition, the cloned genes can serve as substrates for mutagenesis which can lead to alterations in substrate specificity. The genes can also be used to generate strains containing mutant genes by the method of the present invention.
A significant limitation in achieving the above stated goal of cloning antibiotic synthetic pathways is the difficulty in identifying organisms having such pathways. Historically, discovery of antibiotics occurred through evaluation of fermentation broths for anti-bacterial or anti-fungal activity. Such an approach is inadequate in that the biosynthetic pathway would only be implicated by the logical dependence of the product on an underlying biosynthetic pathway leading to its production. U.S. Pat. No. 4,935,340 teaches the use of antibiotic resistance genes as probes for locating macrolide biosynthetic pathways. However the numerous mechanisms whereby resistance to antibiotics is attained and the non-antibiotic utilities of macrolides such as FK506 and avermectin suggests that numerous macrolide biosynthetic pathways could escape detection by the method of U.S. Pat. No. 4,935,340.